Light rays reaching the eye

In summary: P.In summary, when looking at an opaque white object inclined at 45 degrees, if I imagine a point light source next to me and turn it on, I feel that I can see a certain shade of white (grayish). However, when looking at this scene from a physics perspective, there is no ray of light from the light source that hits point P and then reflects to my eye. Instead, when I trace the ray from the eye to point P and toward the reflecting source, I go to infinity.
  • #1
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Assume I am standing in a desert at night (It's pitch dark). I have an opaque white object in front of me inclined at 45 degrees (see diagram). I now turn on a point light source next to me.

http://img199.imageshack.us/img199/8945/lightt.jpg [Broken]

When I imagine the scene I feel I will be able to see point P on the object in front of me, at least a certain shade of white (grayish).

However, when I think of this from a physics point of view, there is no ray of light from the light source that hits point P and then reflects to my eye (considering angle of incidence = angle of reflection).

When I trace the ray from the eye to point P and toward the reflecting source I seem to go to inifnity (there is no wall there to reflect any light as it is a desert).

So in reality will any shade of white on point P be visible to the observer? If yes, then which ray of light does it come from and how does it reach the eye?
 
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  • #2
Angle of incidence only equals angle of reflection for a mirror. Diffuse reflection reflects the light in all directions.
 
  • #3
Is it not possible to have a mirror with colour film over it (just as an indicator that you're seeing point P) or even a mostly opaque surface that doesn't diffuse reflection?
 
  • #4
russ_watters said:
Angle of incidence only equals angle of reflection for a mirror. Diffuse reflection reflects the light in all directions.

Ahhh... but isn't diffuse reflection a consequence of rough surfaces? If the object was very glossy/smooth then would it still be seen?
 
  • #5
But there are lots of other objects around like sand that will catch a little of the point source of light, and then bounce off the glossy screen and into your retina.
 
  • #6
Verminox said:
Ahhh... but isn't diffuse reflection a consequence of rough surfaces? If the object was very glossy/smooth then would it still be seen?
I'm not sure what the exact mechanism is, but you specified white, not mirrored in your first post...
 
  • #7
Note that a point source of light still emits in all directions. Your diagram is incomplete in that it does not show any other rays of light - but there exists a ray of light from that source that will reach the eye. You would see the point of light in the reflection.

What I think you're asking about is a collimated source, such as a laser. That's what you'd need to use in order for your diagram to be representative.

And as we know, the beam form a laser is invisible unless it enters your eye - whether directly, or indirectly through some type of scattering.
 
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  • #8
russ_watters said:
I'm not sure what the exact mechanism is, but you specified white, not mirrored in your first post...
Again, can't it be possible to have a white surface that doesn't diffuse reflection? It would rather act like a mirror.

DaveC426913 said:
What I think you're asking about is a collimated source, such as a laser. That's what you'd need to use in order for your diagram to be representative.

And as we know, the beam form a laser is invisible unless it enters your eye - whether directly, or indirectly through some type of scattering.
Well now I'm convinced.
 
  • #9
DaveC426913 said:
Note that a point source of light still emits in all directions. Your diagram is incomplete in that it does not show any other rays of light - but there exists a ray of light from that source that will reach the eye. You would see the point of light in the reflection.

The only ray of light that I have shown in the diagram is the one which hits point P. There are few rays of light that reflect at such an angle that it enters the eye. But these rays are not bouncing off point P. Note that the surface normal is uniform throughout and all this is assuming angle of incidence = angle of reflection.

Mentallic said:
Again, can't it be possible to have a white surface that doesn't diffuse reflection? It would rather act like a mirror.

Exactly my thoughts. A smooth 'finish' to a glossy material can minimize diffuse reflection.


Anyway, the idea was not going into a debate of what material the object needs to be in order to be invisble. I just wanted to clarify my doubt of how we can see objects for which there may be no obvious ray trajectories using the law of reflection. I realize that the reason we still see these points is because of diffuse reflection from the surface, successive reflections from surrounding objects and perhaps scaterring of light by dust particles in the air.

Thanks a lot everybody :smile:
 
  • #10
Mentallic said:
Again, can't it be possible to have a white surface that doesn't diffuse reflection? It would rather act like a mirror.
I have never heard of a color being associated with a mirror. It sounds like arguing against a definition to me.
Verminmox said:
Anyway, the idea was not going into a debate of what material the object needs to be in order to be invisble. I just wanted to clarify my doubt of how we can see objects for which there may be no obvious ray trajectories using the law of reflection.
If we change that one little phrase "white object" to "perfect mirror" in your first post, then the answer is no, we can't see the light.
 
  • #11
Verminox said:
The only ray of light that I have shown in the diagram is the one which hits point P. There are few rays of light that reflect at such an angle that it enters the eye. But these rays are not bouncing off point P. Note that the surface normal is uniform throughout and all this is assuming angle of incidence = angle of reflection.
What is the significance of point P though? It's not where you will (or should) see the light. (This should be obvious during the day. Your reflective surface will reflect the distant horizon, and it that reflection, you will (and should) see the source of light slightly off to the side).

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  • #12
If we assume perfect situations such that the same diagram the OP posted is in deep dark space, and it is a perfect mirror reflecting the light rays. However, there is a barrier between the observer and light source so that the light cannot travel directly to the observers eye. The reflective surface is also in such a position that it cannot be possible to reach the observer's eye through reflective positions.

Would the observer see nothing at all? Complete darkness?

I'm not sure if this would apply, but just putting it out there - as gravity can bend light, is it possible to bend the light enough so as to bend around the barrier or reflect and bend to reach the observers eye?
 
  • #13
Mentallic said:
If we assume perfect situations such that the same diagram the OP posted is in deep dark space, and it is a perfect mirror reflecting the light rays. However, there is a barrier between the observer and light source so that the light cannot travel directly to the observers eye. The reflective surface is also in such a position that it cannot be possible to reach the observer's eye through reflective positions.

Would the observer see nothing at all? Complete darkness?
Yep, complete darkness. If no light rays can reach the observer's eye either directly or by reflection, the observer will not see any light.

Mentallic said:
I'm not sure if this would apply, but just putting it out there - as gravity can bend light, is it possible to bend the light enough so as to bend around the barrier or reflect and bend to reach the observers eye?
Sure, but depending on the distances involved you might need a black hole to do it. There's no object on Earth, or even in the solar system, that can bend light enough to detect without very sensitive measuring instruments.
 
  • #14
Thanks for clearing that up for me diazona :smile:
 
  • #15
I have corrected the OP's original diagram.

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What is the source of light rays reaching the eye?

The source of light rays reaching the eye can vary depending on the environment. In natural settings, the sun is typically the primary source of light, while in artificial settings, light can come from sources such as light bulbs, screens, or other electronic devices.

How do light rays reach the eye?

Light rays enter the eye through the cornea, a clear outer covering that protects the eye and helps to focus light. Then, the light passes through the pupil, which is the black circular opening in the center of the iris. The iris adjusts the size of the pupil to control the amount of light that enters the eye. Next, the light passes through the lens, which further focuses the light onto the retina. The retina contains specialized cells called photoreceptors that convert light into electrical signals and send them to the brain for processing.

What happens to light rays once they reach the eye?

Once light rays reach the eye, they are focused onto the retina by the cornea and lens. The photoreceptors in the retina then convert the light into electrical signals that are sent to the brain through the optic nerve. The brain then processes these signals to create the images that we see.

Can light rays be harmful to the eye?

Exposure to excessive amounts of light can be harmful to the eye. This is known as phototoxicity and can lead to damage of the retina or other structures of the eye. It is important to protect the eyes from prolonged exposure to bright light, especially sunlight, by wearing sunglasses or limiting screen time.

How does the eye perceive color from light rays?

The eye perceives color through the photoreceptors in the retina. There are two types of photoreceptors: rods, which are responsible for low-light vision, and cones, which are responsible for color vision. There are three types of cones that are sensitive to different wavelengths of light, which allows the eye to perceive a range of colors. The brain then processes these signals to create the perception of color.

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